It is known that the Variable Speed Constant Frequency (VSCF) induction generators have a potential application for conversion of fluctuating wind energy to a constant frequency active electrical power, which has been made through different methods using VSCF induction generator since early 1970s. Normally, two types of systems are known and are used for achieving constant frequency output or power supply from a variable speed primemover.
In the first system, known as the fixed speed system, the variable speed from the primemover is converted to fixed speed using gear system, which in turn drives the fixed speed generator to yield constant frequency power supply.
In the second system, known as the variable speed system, the generator is directly connected to the variable speed primemover and the generator yields a constant frequency AC power supply through the use of electronic power converters.
In both the systems, the squirrel cage and slip-ring type induction machines are used. The second system has attracted more attention since the complex mechanical gear arrangement can be disposed off, increasing reliability. In the second system however, both types of machines require electronic power converter for generating constant frequency electrical power under varying speed condition of the prime mover. The squirrel cage induction generator requires electronic power converter of 100% rating introduced between electric power grid and generator stator winding while the slip-ring or wound rotor induction generator requires electronic power converter of partial rating introduced between rotor winding and electrical grid, with its stator winding directly connected to the electrical grid. Such a slip-ring machine arrangement is known as a Doubly Fed Induction Generator (DFIG).
The major advantage of DFIG, which is already made popular, is that the electronic power converter has to handle only a fraction (20 to 30%) of the total system power. Recent research and advancement in this area has led to the development of induction machines using two sets of stator winding for converting power from a fluctuating speed primemover to a constant frequency electrical active power, but still retaining the need for electronic power converters. The two separate stator windings were placed in the same slots but wound for two sets of balanced polyphase windings with two different numbers of poles such that the flux created by one set of winding does not interfere with that of the other. One of the windings called as “power winding” is connected to the grid directly while the second winding called as “control winding” is also connected to the grid through an electronic power converter. Thus, existing technology in this area utilizes some form of electronic power converter in association with a type of induction machine to generate fixed frequency active power to the grid, when driven by a variable speed primemover.
In the known machines, due to the incorporation of electronic power converters for generating constant frequency electrical active power from fluctuating speed prime movers, the following problems were experienced:                1. Extra cost: For larger wind turbines of 3 to 4 MW range, the converter rating (around 25%) is quite large, thus increases the cost of converter (its associated transformers and power filters) and associated control circuits which are more complicated. This ultimately increases the cost of energy intended to be produced.        2. Degradation of Power Quality: The electronic power converter injects switching harmonics in the connected grid and degrades the power factor of the bus.        3. System Stability Problem: The converter introduced in DFIG system, often loses control of the rotor current and would ultimately go out of control due to disturbance in power line especially in voltage sag conditions.        4. Reduced Life of the Bearings in the Generator: In DFIG system, the introduced Pulse Width Modulated (PWM) converter damages the bearings of the generator due to the flow of earth current through such bearings, produced by interference created by the converter.        5. Increased Ownership cost: The electronic power converter being an active element is susceptible to damage (particularly due to voltage surges) than any passive element. So the chance of damage is more and ultimately increases the ownership cost and for replacement of the required accessories.        6. Losses in the Converter System: The converter losses in Variable Speed Induction Generator (VSIG) system is around 2.5% of maximum shaft power and the same is around 1% for DFIG system. Additional losses will be incurred in the transformer usually used in existing converter systems.        
To overcome the aforesaid disadvantages associated with the prior art machines having electronic power converters in existing systems for achieving constant frequency electrical power from variable speed prime mover, the applicants have developed a simple dual stator and rotor winding induction generator that can produce constant frequency electrical active power even if the speed of the prime mover changes, without using any electronic power converter.